This invention relates to an apparatus for thermally cooling semiconductor wafers and, more particularly, relates to an apparatus for mechanically clamping a semiconductor against a pliable thermally conductive surface to permit the cooling of the wafer.
In the processing of semiconductor wafers, e.g., in order to fabricate integrated circuits, it is sometimes necessary to subject the wafers to elevated temperatures. For the diffusion of impurities, the growth of epitaxial layers, annealing of metal-semiconductor contacts, and the like, such elevated temperatures are desirable. However, at many points in processing it is undesirable to expose the wafer to elevated temperatures since gratuitous diffusion beyond prescribed limits, as well as the segregation of impurities at epitaxial interfaces is not desired. Also, intermediate photoresist layers may be affected at such temperatures. This problem is enhanced in the fabrication of large scale integration (LSI) and very large scale integration (VLSI) devices since a large number of processing steps must be used in sequence; in particular, near the end of the processing sequence there are large numbers of impurities, conducting layers or insulating layers in place and it is not desirable to disturb these physical features by thermal treatment. Thus, it is desirable to expose semiconductor wafers to elevated temperatures only when a process step positively requires it and, if necessary, to be able to cool a semiconductor wafer to prevent elevated temperatures from being attained.
Ion implantation has become a standard technique for introducing inpurities into semiconductor wafers. See, e.g., P. D. Townsend, et al., Ion Implantation, Sputtering and their Applications, pp. 262-263 (1976). Impurities are introduced into the bulk of semiconductor wafers by using the momentum of energetic ions as a means of embedding them in the crystalline lattice of the semiconductor material. The kinetic energy of the impinging ions determines the depth of penetration. The depth of penetration is normally chosen to be the appropriate depth at which the impurity will be permanently fixed. However, on occasion, thermal diffusion may be used as an adjunct to ion implantation, as in a low energy predeposition machine; in this situation, however, the subsequent thermal diffusion will be a specific process step with known time and temperature characteristics.
As energetic ions impinge on a semiconductor wafer and travel into the bulk, heat is generated by the atomic collisions. This heat can become significant as the energy level is increased. Also, it is sometimes desirable for economic reasons in commercial applications to have a high throughput; consequently, as high an ion flux as does not do physical damage to the structure of the semiconductor material of the wafer will be used. Large amounts of heat may thus be generated. As set out above, this heat is undesirable.
Previous attempts to cool semiconductor wafers which are undergoing ion implantation include effecting intermittent exposure by scanning either or both the ion beam or the wafer (thereby limiting throughput), providing an actively cooled metal plate, coated with grease or oil, for the semiconductor wafer to rest upon, and applying an electrostatic force to hold a wafer against a slightly compressible surface on an actively cooled plate (see L. D. Bollinger, "Ion Milling for Semiconductor Production Processes", Solid State Technology, November 1977). These prior art techniques and devices have proven to not be wholly effective at cooling semiconductor wafers when high ion fluxes or energy levels are experienced.
It is therefore an object of the present invention to provide an apparatus for permitting the effective cooling of semiconductor wafers during ion implantation.
It is another object of the present invention to provide an apparatus for mechanically clamping a semiconductor wafer against a thermally conductive pliable material adhered to a convexly curved platen for rapid dissipation of thermal energy.
It is a still further object of the present invention to provide a device for thermally cooling semiconductor wafers which effects good thermal transfer without interference from any surface irregularities in the semiconductor wafers